The present invention concerns a filament type sensor for determining a static or dynamic characteristic of a gas environment such as the air, for example, a method of fabricating it, and applications of the sensor primarily to the detection of oxidizable gases but also to gas chromatography (detection of ionizable gases) and fluid flowrate measurement.
A filament type sensor of this kind has a resistive element within a filament adapted to exchange heat with the environment and an interface area adapted to react with the environment in a physico-chemical process. In the broadest possible sense of the term, this process includes catalysis of combustion, adsorption, ionization, simple thermal exchange, as well as others which influence an electrical characteristic of the interface area, i.e., temperature or resistance, voltage, current, etc., according to the characteristic of the environment to be determined, for example, concentration, flowrate, etc. The interface area can be the external portion of the resistive element, or a catalyst film heated by conduction, or a separate electrode.
Some sensors of this kind are based on measuring the heat exchanged (detection of combustible gases, flowmeter, etc) and may be characterized as calorimetric sensors. There are also various filament type sensors having the common feature of measuring a concentration, based on various phenomena; for example, measurement of the heat exchanged in the case of detecting combustible or oxidizable gases, or measurement of the quantities of ions captured by an electrode in gas chromatography. Filament type sensors are, therefore, of very diverse kinds, both with regard to the physico-chemical phenomenon on which their operation is based and with regard to the nature of the parameter to be measured.
Although the remainder of this description refers for the most part to the detection of an oxidizable gas in a gas environment such as the air, in the field of explosimetry, for example, this is a preferred application and is not limited to the invention.
A known way of detecting an oxidizable gas in the air uses a filament, usually of platinum, heated by the Joule effect, i.e. by the passage of an electric current. The oxidizable gas contained in the surrounding air is oxidized by catalysis in contact with the filament, so that the latter is further heated. The resulting temperature variation causes a variation in the resistance of the filament, which is measured directly or indirectly to obtain the concentration of the oxidizable gas in the air. These filament-based detectors are largely hand-made. They, therefore, suffer from a lack of reproducibility and high cost. Their low electrical resistance and their low surface area/volume ratio make it necessary to operate them at high temperatures, for example, about 1000.degree. C.
Other oxidizable gas detectors are based on catalytic beads; they are formed by a metal detector, of platinum, for example, coated with alumina doped with a catalyst, and resemble a small pearl. These detectors age less rapidly, as the associated combustion temperature is lower. However, these beads have the disadvantage of significant drift in sensitivity, reduced stability and an increased response time as compared with filaments.
A third type of oxidizable gas detector is based on semiconductor metal oxides doped with a catalyst. These detectors are formed by a metal heating element which heats an insulative material (alumina, for example) sleeve onto which is deposited a film of semiconductor material whose resistance variations are measured. These detectors are sensitive to any gas that can be absorbed onto the surface of the semiconductor. They have a relatively long response time, however, and the further disadvantage of high electrical power consumption; also, the effects of humidity are not compensated.
The invention is directed to alleviating the aforementioned disadvantages by improving reproducibility and by reducing thermal losses from the filament by conduction, while also reducing manufacturing costs.